1. Field of Invention
The invention relates to a Coriolis force type flow meter and, in particular, to a Coriolis force type flow meter that uses an optical interferometer as its measuring device.
2. Related Art
In many manufacturing processes or applications that require flow control, the first thing that one has to do is to precisely measure and control the flux in order to properly make desired products. For example, in the biochemical technology the formation of a chemical compound requires two or more substances mixed in a specific ratio. Another example is that in motor engines, the gasoline and air have to be mixed in an appropriate ratio to achieve an optimal efficiency.
Currently, most flow meters utilize the changes in pressure, temperature or acoustic wave propagation of the fluid flowing through a tube to determine the flux. According to different measuring methods, the flow meters can be categorized as the thermal, pressure-difference and ultrasonic types. The physical quantity obtained from the above-mentioned flow meters is normally the flow speed (meters per second) or volume flux (cubic meters per second). Once the density of the fluid is known, the mass flux of the fluid can be readily calculated.
However, the fluid flow measuring method of the above-mentioned flow meters is indirect. The precision of the measurement is very likely affected by changes of the fluid properties, such as its temperature, pressure, density, viscosity, and homogeneity. Moreover, the precision may also be affected by the change in the distribution of the flow field.
To conquer the above drawbacks, Micro Motion, Inc first developed a flow meter that utilizes the principle of Coriolis forces in 1997. By directly or indirectly measuring the Coriolis force generated by the fluid flowing inside a rotational tube, one is able to obtain the mass flux of the fluid. This type of flow meter can directly measure the fluid flux inside the tube. The best advantage is: a high precision measurement can be achieved without being affected by changes in the fluid properties. Nevertheless, such a flow meter also has its shortcomings. In order to measure the tiny variation in the flow field caused by the Coriolis force, the size of the flow meter has to be large enough. A relatively complicated measuring device has to be used in order to achieve the high precision requirement. Therefore, the manufacturing cost of the flow meter increases and the product is not suitable for measurements in small flux fluid flows.
The method disclosed in the U.S. Pat. No. 6,412,355 uses basically the same idea as that of Micro Motion, Inc. However, the measurement is made through electrical signals from two different points in a tube. The flux inside the tube is obtained from its relation with the phase difference and the vibration frequency. As in the previous case, the size of this type of flow meter is larger and the device has a rather complicated structure. Therefore, it is not suitable for low fluid flow measurements either.
In the conference paper xe2x80x9cA Coriolis Mass Flow Sensor Structure in Siliconxe2x80x9d presented in 1996 IEEE Meeting, Enoksson et. al. proposed a new method of measuring the fluid flux by first projecting a laser beam on a double-loop tube in motion and the computing a rotation angle from the measurement of the position change of the reflected light on a photon detection apparatus. However, both positioning and calibration of the whole optical measuring system are not easy, the fact of which in turn affects the sensitivities. Therefore, one has to try to obtain compensations from other aspects, such as increasing the input voltage of the stimulator.
In view of the difficulties of using the above-mentioned flow meters to make measurements, an objective of the invention is to provide a Coriolis force type flow meter that uses a Fabry-Perot interferometer to measure the fluid flow inside a tube. Since the sensitivity of this type of optical interferometers can reach the micrometer order, it is ideal for measuring the minute flux changes inside a tube. AS the setup and calibration of the Fabry-Perot interferometer is not difficult at all, the manufacturing cost of the measuring device can be lowered. In contrast, such advantages increase the competition power of the disclosed Coriolis force type flow meter with others.
The Coriolis force type flow meter according to the invention has a substrate with stimulating electrodes for providing an electrostatic force and small holes symmetrically distributed on both sides of the stimulating electrodes. The substrate is installed with a symmetric rectangular loop tube, whose back end allows fluid to enter and/or leave. Its front end is installed above the stimulating electrodes. Driven by the electrostatic force provided by the stimulating electrodes, the rectangular loop tube starts bending vibrations.
The front end of the rectangular loop tube has through holes that are also symmetric, corresponding to the above-mentioned small holes. Several reflective mirrors are installed in the small holes and the through holes of the loop tube. A light source is provided above the through holes of the loop tube. A photo probe is installed under the small holes of the substrate.
The light emitted from the light source passes the reflective mirrors in the through holes and small holes. The photo probe extracts the interfered optical signals. After specific calculations, the fluid flow inside the rectangular loop tube can be obtained.